ISO/TR 26946-2011 pdf free.Standard method for porosity measurement of thermally sprayed coatings.
This Technical Report describes a method for characterizing the porosity of thermally sprayed coatings by metallographical examination.
This method is particularly applicable to oxide coatings, such as Al203, Zr02 and Ti02, produced by plasma spray. It also considers the purposes to test the size, shape and density of pores for thermally sprayed coatings.
2 Purpose
The main purpose of porosity measurement is to determine the quality of a thermally sprayed coating and its freedom from porosity, particularly on those areas of the significant surface that demand a functional requirement.
This Technical Report provides a standard process that is suitable for determining the porosity of thermally sprayed coatings, as part of the total quality assurance programme.
This Technical Report is also intended to provide a standard way to present the porosity of thermally sprayed coatings.
3 Classification
The microstructure of a thermally sprayed ceramic coating is characterized by the existence of various pores, microcracks, splat boundaries and unmelted particles, because of the nature of the process. Although different terms are used, both the pores and the microcracks are volumetric spaces, which are free from coating material. The pores can be divided into closed pores, open pores and micropcwes. Closed pores appear as isolated clustered voids in the coating and have no connection with the surface; open pores appear as the same voids but have a connection with the atmosphere, either directly or from one pore to another, micropores are either closed or open pores which show dimensions only detectable on a microscopic scale. The difference between pores and microcracks lies mostly in their aspect ratios (ratio of the major axis over the minor axis), so, they are collectively treated as pores. The fraction of volumetric space covered by the pores in thermally sprayed coatings is defined as porosity.
4 Principle
The porosity of thermally sprayed coatings is determined by preparing an area of the inspected coating with a cross-section of high microscopic surface quality, which can be viewed using a light microscope or a scanning electron microscope (suggested). A quantitative assessment of the porosity of the inspected coatings is carried out by using an image analysis technique on the microscope.
5 Apparatus
The following equipment is necessary for the porosity measurement of thermally sprayed coatings.
5.1 Cut-off wheels (recommended) or diamond wire or high pressure water-jet cutting equipment. (according to equipment in existence) for sectioning coating samples to a proper size with minimal damage.
5.2 Cleaning apparatus, with ultrasonic equipment.
5.3 Mounting equipment.
5.4 Grinding and polishing equipment, (semi-automated or automated grinding/polishing machines are
recommended for consistent reproducibility).
5.5 Scanning electron microscope (recommended) or light microscope. for viewing the inspected
sample on a cross-section and obtaining digital images.
5.6 Computer, with analysing software for porosity evaluation on digital images.
All equipment should undergo regular maintenance and calibration to assure reliability and repeatability of the measurement. At the same time, all metallographic personnel should have the proper training to allow them to perform the required functions and analyses.
6 Metallographic preparation
6.1 General
Metallographic preparation of thermally sprayed coatings is critical for the porosity results. The requirements for detail and monitoring will vary from system to system, depending upon the degree of automation in the preparation. The basic steps for the preparation are given in 6.2 to 65.
6.2 Sectioning
If sectioning is required, two commonly used methods are abrasive wheel cutting or diamond wire cutting. The first one, which is comprised of a diamond or boron nitride saw (more effective in this purpose) that breaks down readily exposed fresh cutting surfaces, is usually best for a wide range of coatings. Sectioning should be done with the culling force from coating to substrate and minimal clamping pressure on the sample. It will be better to secure the specimen for sectioning with a soft cushion, such as wood, if possible. The sectioning wheel should be as thin as possible to minimize damage, which must be removed in subsequent steps. Minimum pressure should be applied on the wheel to minimize possible overheating, with cooling by water if possible. The length of the test specimen should be greater than 1 cm. At least five test specimens should be taken from each sample in different positions.
6.3 Cleaning
Cleaning is an important step for removing all contaminants from the surface of the specimen Three methods or any combination are recommended.ISO/TR 26946 pdf download.